MIGRATIONS OF COD 
81 
marked fish present on the tagging ground due to the fishery, natural deaths, and 
emigrations, which, if they could be closely estimated, would add considerable 
accuracy to the calculations. 
In addition certain basic assumptions must be made, for whether the estimated 
population of the tagging grounds can be extended to include all the grounds on 
Nantucket Shoals depends on (a) whether we are justified in assuming that cod are 
equally abundant on all the ground in this region which appears to be suitable for 
them, and ( b ) whether the estimate of the total area of both the tagging grounds and 
all of the shoals is correct. 
As for the first assumption, we have no definite data as to the density of cod on 
Nantucket Shoals except for the tagging ground, but it is known that fishing vessels 
make good catches in places other than this ground; in fact, most of the commercial 
catch of cod on the shoals is taken along the eastern and southern parts where no 
marking has been done. If, therefore, all the bottom on the shoals which supports 
cod, containing as it does some areas where the fish are concentrated and others where 
they are sparse, be averaged, it is probable that the density of cod in the region desig- 
nated as the “tagging ground” is very much the same as that on any other part of 
Nantucket Shoals of about the same area and average depth. 
With regard to the area of the tagging ground, it is estimated to comprise about 
one-fourteenth of the total, for almost all of our fishing there was done along a strip 
about 20 miles long and 2/ miles wide. The total area of cod bottom on the shoals 
is estimated at about 700 square miles. 
An estimation of the size of the cod population on the tagging ground must 
depend largely on the number of marked fish that were present there, available for 
recapture, during the spring to fall of the years 1923 to 1929. Unfortunately, it is 
virtually impossible to gage this accurately, for we have scarcely any data that throw 
light on the degree of gain or loss in the number of marked fish present from year to 
year. We can, however, obtain some idea of what the minimum population may 
have been. 
A hypothetical example may make this clear. Suppose, for instance, that all the 
cod tagged on Nantucket Shoals during one year disappeared by the next, but that 
virtually all these fish were available for recapture on the tagging ground during the 
summer when they were tagged. (We have some basis for making this last assump- 
tion, for most of the fish remain localized during the summer and the chief losses 
would be caused by natural deaths and by recaptures made by the fishery; these would 
probably be small in so short a time interval.) Under such circumstances, if 5,000 
cod were caught and tagged on the shoals from the spring to the fall of one year you 
might say that, taking the whole period as a unit, an average of about 2,500 of them 
were available for recapture there by a tagging vessel during the course of its fishing 
on the shoals that year. If 20 of these fish were recaptured by the time the catch 
of 5,000 had been completed, then we might conclude that 1 fish out of each 250 
within that area designated as the “tagging ground” had been marked. The total 
population of the tagging grounds would be, therefore, two hundred and fifty times the 
number of marked fish available, or 250 X 2,500, which gives a result of 625,000. 
